Carlson T A, Martin G B, Chelm B K
Department of Biochemistry, MSU/DOE Plant Research Laboratory, East Lansing.
J Bacteriol. 1987 Dec;169(12):5861-6. doi: 10.1128/jb.169.12.5861-5866.1987.
Bradyrhizobium japonicum induces the formation of nitrogen-fixing symbiotic root nodules on soybean plants. The B. japonicum genome encodes two isoforms of glutamine synthetase (GS). One form, GSI, encoded by the gene glnA, is similar in structure and activity to the enzyme found in all other bacteria. The second form, GSII, encoded by glnII, is structurally related to the eucaryotic enzyme. Genetic analyses indicate that glnA or glnII alone is sufficient to provide glutamine prototrophy, whereas the double mutation glnA glnII produces glutamine auxotrophy. The glnA gene is transcribed from a single promoter that has a structure most similar to that of the bacterial consensus promoter. The level of transcription of glnA is not specifically affected by nitrogen limitation of growth. The glnII gene is also transcribed from a single promoter; however, this promoter has structural features characteristic of promoters controlled by the nitrogen regulation system. In contrast to glnA, physiological studies indicate that glnII transcription is regulated in response to nitrogen source availability. Under aerobic growth conditions, expression of glnII is induced when growth is limited by nitrogen source depletion as expected for regulation by the nitrogen regulation system.
日本慢生根瘤菌能在大豆植株上诱导形成固氮共生根瘤。日本慢生根瘤菌的基因组编码两种谷氨酰胺合成酶（GS）同工型。一种形式，即由glnA基因编码的GSI，其结构和活性与在所有其他细菌中发现的酶相似。第二种形式，即由glnII编码的GSII，在结构上与真核酶相关。遗传分析表明，单独的glnA或glnII足以提供谷氨酰胺原养型，而双突变体glnA glnII则产生谷氨酰胺营养缺陷型。glnA基因从一个单一启动子转录，该启动子的结构与细菌共有启动子的结构最为相似。glnA的转录水平不受生长氮限制的特异性影响。glnII基因也从一个单一启动子转录；然而，该启动子具有受氮调节系统控制的启动子的结构特征。与glnA相反，生理学研究表明，glnII的转录受氮源可用性的调节。在有氧生长条件下，当生长受到氮源耗尽的限制时，glnII的表达如预期的那样受氮调节系统的调节而被诱导。
